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Measuring tools
• Modern engines use thinner casting and require
more precise measuring
Selectively fit engines
• All parts are measured and marked according to size
• Then selectively assembled for optimum clearance
Copyright 2003 Gary Lewis - Dave Capitolo
Tolerance limits
• The minimum and maximum allowable diameters
Specified clearance
• Difference in diameters for
mating parts
• Specified clearance of
replacement part should
be used
Example of tolerance limits for crankshaft and housing bore
• Standard shaft size
1.9995/2.0005 in.
• Housing bore diameter 2.1245/2.1250 in.
• Difference between shaft OD and bearing ID
• Minimums and maximums
Maximum bearing inside diameter
Maximum housing bore
+2.1250 in.
Bearing thickness
-.0615 in.
Bearing thickness
-.0615 in.
Max bearing inside diameter 2.0020 in.
Minimum bearing inside diameter
Minimum housing bore
+2.1245 in.
Bearing thickness
-.0615 in.
Bearing thickness
-.0615 in.
Min bearing inside diameter 2.0015 in.
Minimum bearing clearance
Minimum bearing ID
Maximum crankshaft diameter
Minimum bearing clearance
+2.0015 in.
- 2.0005 in.
.001 in.
Maximum bearing clearance
Maximum bearing ID
Minimum crankshaft diameter
Maximum bearing clearance
+2.0020 in.
- 1.9995 in.
.0025 in.
Example of valve stem clearance
Valve stem diameter
Stem-to-guide clearance
.3405 - .3415 in.
.0015 - .0025 in.
To find valve guide diameter
Minimum clearance
Maximum stem diameter
Valve guide diameter
.0015 in.
+.3415 in.
.3430 in.
Maximum clearance
Minimum stem diameter
Valve guide diameter
.0025 in.
+.3405 in.
.3430 in.
Thermal expansion
• Engines are assembled at room temperature
• An error at room temperature may cause problems at
operating temperature
Coefficient of thermal expansion
Iron and steel
.000006 in.
.000012 in.
Example from book
Pin diameter
Pin bore diameter
Clearance at room temp
Pin diameter with 250° F change
Pin bore with 250° F change
Clearance with 250° F change
Heat during machining
Rough and finish machining
Measuring tools at room temperature
Units of measure for linear measurement
1 mm
.1 mm
.01 mm
.001 mm
To convert inches to millimeters
Inches times 25.4 or
Inches divided by .03937
To convert millimeters to inches
Millimeters divided by 25.4 or
Millimeters times .03937
Measuring tools
• Outside micrometer
• Inside micrometer
• Depth micrometer
• Telescoping gauge
• Split ball gauge
• Dial indicator
• Dial bore gauge
• Calipers
Outside micrometer
• Screw pitch of 1/40” or .025” for inch micrometer
• Screw pitch of .50 mm for metric micrometers
• One inch of travel
• Resolution limit of .0001” for inch micrometers
• Resolution limit of up to .001 mm for metric
• Calibrated with standards or gauge blocks
Depth micrometer
• Used for measuring deck clearance, stepped flywheels
• Screw pitches same as outside micrometer
• One inch of travel
• Resolution limit of .001” for inch micrometers
• Scales are read in reverse order
• Calibrated with gauge blocks and surface plate
Inside micrometer
Used for measuring cylinder bores, housing bores
Screw pitches same as outside micrometer
One inch of travel
Used more often with ‘transfer’ measurements
Transfer measurement
• Adjust inside micrometer to size
• Measure with an outside micrometer
• Other tools include telescoping and split ball gauges
Dial indicators
Resolution limits printed on dial face
Used for checking straightness of cams and cranks
Read TIR (total indicator reading)
Check out-of-roundness first
Dial bore gauge
Used for cylinder bores and main bores
Uses ‘comparative’ measurement
Set up in a setting fixture or outside micrometer
Measure differences in the size that was set
• Dial and digital calipers
• Lack of rigidity limits accuracy to .002”
• Easy to read
Surface finish
• Crank journals 10 AA micro inches
• Head and block deck surfaces 20–60 AA micro inches
• Cylinder bores 10-15 AA for moly rings
25-30 AA for iron and chrome
Fastener terminology
Major diameter - Largest thread diameter
Minor diameter - Smallest thread diameter
Pitch - Distance between threads
Threads per inch - Number of thread in a linear inch
Grade - A system of grading fastener strength
Threaded fasteners
• Bolts, machine screws, and capscrews stretch
when tightened
• Fasteners have an elastic limit, then fracture occurs
• Below elastic limit, fasteners can be tightened repeatedly
Necked down bolt
• Example of a bolt that has been necked down. Tightening
past its elastic limit
Fastener strength
Cross section of threads
• Minor diameter effects strength
• As pitch increases on external threads, crossection
• As pitch increases on internal threads, crossection
Fastener strength
Heat treatment
• Low strength fasteners are made of low carbon mild steel
• High strength fasteners have added carbon for heat treating
• SAE fasteners have marks for strength
• Metric fasteners have numbers for strength
Thread identification
• Outside diameter of the threads with a micrometer
• Measure pitch with thread pitch gauge (threads per inch)
• For the machine screw 10-32
Diameter: 10 x .013 + .060 = .190 in.
Pitch: 32 threads per inch = .031 in.